1. Field of the Invention
This invention relates to tin coated copper based electrical conductors having a reduced rate of copper/tin intermetallic formation. More particularly, a diffusion barrier is disposed between a copper base substrate and a tin base coating. The diffusion barrier is a multi-layer structure with at least one of the constituent layers being copper base and having a thickness dependent on the thickness of the tin base coating.
2. Background of the Invention
Electrical conductors, including connectors such as sockets and plugs, leadframes and printed circuit boards are typically formed, at least in part, from a copper base alloy substrate that provides good electrical conductivity. Throughout this patent application, the word "base" is used in its common sense in the metallurgical arts, namely that the substrate is predominantly, or more than 50% by weight, copper. When exposure to elevated temperatures is anticipated in service, the electrical conductor substrate is typically formed from a copper base alloy having high strength and a resistance to stress relaxation.
Stress relaxation resistance is recorded as a percentage of the stress remaining after a sample of the copper base alloy is preloaded to a set percent of the yield strength, typically 80% in a cantilever mode per ASTM (American Society for Testing and Materials) specifications. The strip is then heated, typically to 125.degree. C. for a specified number of hours, typically up to 3000 hours, and retested periodically. The higher the stress remaining at each retest, the better the utility of the specified composition for spring applications.
To reduce the elevated temperature oxidation, tarnish and corrosion of the copper base substrate, as well as to enhance solderability and to maintain electrical performance, a coating is frequently applied over the substrate. Typical coatings include nickel, palladium/nickel alloys, tin and tin base alloys. To minimize cost, tin is frequently used.
At elevated temperatures, the copper and the tin interdiffuse to form copper/tin intermetallic compounds. The formation of the intermetallic compounds reduces the amount of unreacted or free tin on the surface. The absence of a surface layer of free tin leads to an increase in electrical contact resistance and degrades the corrosion resistance, solderability and other performance characteristics of the electrical connector.
To reduce the rate of intermetallic formation and thereby reaction of the free tin, a barrier layer is interposed between the copper base substrate and the tin base coating layer. A publication by Kay et al. appearing in the Transactions of the Institute of Metal Finishing, Volume 59, 1979, at page 169, discloses that barrier layers such as nickel, cobalt and iron, as well as alloys such as tin/nickel, copper/tin and nickel/iron, are disposed between a copper base substrate and a tin coating to reduce the formation of intermetallic compounds.
Barriers formed from multiple layers of different metallic constituents are also known. For example, U.S. Pat. No. 5,384,204 by Yumoto et al. discloses a copper base tape automated bonding (TAB) tape that is coated with a tin or a tin/lead alloy film. A diffusion barrier is disposed between the substrate and the film. The barrier may be a single layer or multiple layers and is formed from materials that will not diffuse into an adjoining structure (i.e. into the substrate or into the coating).
In addition to electrical connectors exposed to high temperature during service life, such as automotive connectors, other applications that expose tin base coatings to high temperatures are the manufacture of printed circuit boards and the forming of leadframes for electronic packages.
The outer lead ends of copper base leadframes are typically coated with a solder, for joining the leadframe to the circuit traces of a printed circuit board (PCB). The leadframe may be exposed to a number of elevated temperature cycles (in excess of 175.degree. C.) since different components may be joined to the PCB at different times and re-work may be required. Each elevated temperature cycle causes the growth of the copper/tin intermetallic at solder joints. To retain free solder, it is conventional to deposit a very thick solder layer (in excess of 400 microinches) on both the outer leads of the leadframe and on the circuit traces of the PCB.
There are problems associated with thick solder coatings, firstly an increase in cost. In addition, for the leadframe, the thick solder layer increases the leadframe thickness and causes problems with trimming and forming. For the PCB, the thick solder layer creates a planarity problem. These problems are becoming more pronounced with finer pitch packages and circuit traces where solder bridging is causing electrical short circuits.
While the prior art barriers are effective to reduce the rate of intermetallic formation, the reduction in rate is not sufficient for applications that require a minimum amount of free tin to remain after an extended thermal exposure such as 125.degree. C. for 3000 hours. Accordingly, there remains a need for an enhanced barrier to be interposed between a copper base substrate and a tin base coating.